Aviation safety investigations & reports

Aerospatiale AS.332L, VH-BHY

Summary

History of the flight

At approximately 1715 on 29 August 2003, the crew of a
Eurocopter AS332L 'Super Puma' helicopter, registration VH-BHY,
being operated on an offshore commuter flight from Karratha,
Western Australia, reported feeling a sudden airframe jolt,
followed by a pitch up, roll, and a left yawing motion. Finding
they had lost tail rotor control, the crew stabilised the aircraft
using pitch and roll control inputs, before declaring a MAYDAY to
air traffic services. After assessing the helicopter's condition
and vibration levels, the crew elected to return to Karratha where
a run-on landing could be performed. The MAYDAY condition was
downgraded to a PAN and, after assessing the helicopter's
performance during a precautionary approach, a safe run-on landing
was conducted.

The aircraft was carrying a flight crew of two and six
passengers who were uninjured.

Damage to the aircraft

Damage to the helicopter was limited to the tail rotor pitch
change assembly and the tail boom lower keel fairing, which had
pulled out several attachment screws. During the initial
post-incident inspection, the operator's ground maintenance
personnel found the nut and lock washer disconnected from the servo
end of the pitch change rod, allowing the rod to move freely within
the servo body. The nut and washer were subsequently found in the
tail structure beneath the tail rotor drive shaft. The rod (P/N
332A33-0043-00) had sustained circumferential gouging and scoring
around the surfaces adjacent to the inboard side of the pitch
change spider bearing (P/N 330A33-9903-20). The bearing itself
showed evidence of gross mechanical failure, with break-up of the
ball cage and dislodgement of the outboard and inboard seals. The
outboard bearing retention nut and lock washer remained in-place
and secure (figure 2).

The Super Puma helicopter tail rotor control was effected by a
hydraulic servo-actuator that applies control force to the tail
rotor blades via a central shaft and spider assembly. A locking nut
and lock washer secured the actuator to the shaft, assembled to a
nominal 266 - 443 pound-inches ( 30 - 50 Newton-metres) dry torque.
At the spider end, the connection was similar, with a nominal dry
torque of 115 - 266 pound-inches (13 - 30 Newton-metres). The Super
Puma tail rotor turns in a counter-clockwise direction when viewed
from the right side of the aircraft. The securing nut on the servo
end of the pitch change shaft had a conventional thread, while the
nut on the spider end of the rod had a left-hand thread. Figures 3
and 4 illustrate the tail rotor assembly and pitch change shaft
location.

Maintenance history

The failed tail rotor bearing was first fitted to VH-BHY in June
2000, as part of a complete replacement tail rotor gearbox (TRG)
assembly. The gearbox, including bearing, had 199 hours time since
overhaul (TSO) when installed. Replacement of the pitch change
bearing is normally carried out during gearbox overhaul, however
documentation to confirm that action was not available to the
investigation.

In June 2003, maintenance action was carried out on the gearbox
in response to elevated lateral vibration levels recorded by the
helicopter's integrated health and usage monitoring system (IHUMS).
Subsequently, on 7 August 2003, the tail servo was replaced after
the discovery of leaked hydraulic fluid inside the boot between the
tail rotor hub and the pitch change spider. It was evident that the
fluid had travelled from the tail servo, through the tail rotor
drive shaft and into the boot, bringing the fluid into close
proximity with the inboard end of the tail rotor pitch change shaft
bearing. The gearbox and assembly had accrued 1,888 hours TSO at
that time. During the weeks following the hydraulic leak, the pitch
change shaft bearing was inspected as required by service bulletin
SB05-00-29 Rev. 3 and accepted for further service. At the time of
failure on 29 August 2003, the TRG and pitch change shaft bearing
had operated for 1,959 hours since overhaul.

Bearing failure

The bearing fitted to the tail rotor pitch change assembly on
VH-BHY was a single race, fully sealed ball bearing, manufactured
by SNFA, France. The bearing carried the following identifying
marks:

330A33990320 8020141 SNFA FRANCE V80I24K14

ATSB laboratory examination of the bearing confirmed the
mechanical failure and break-up of the bearing cage, allowing the
circumferential movement of the balls relative to each other and
the resultant development of abnormal race loading and frictional
conditions (figure 5). The bearing internal surfaces were dry and
in most places covered with an adherent black compound (figure 6)
that was sampled for later analysis. There was no evidence of any
viscous bearing grease remaining within the bearing confines. All
rolling contact surfaces of the bearing showed bruising and
particle indentation damage (figure 7), however there was no
indication of spalling or other rolling contact fatigue type
breakdown. None of the bearing components showed evidence of gross
overheating or frictional seizure. The bearing cage showed gross
levels of wear and metal loss in areas exposed to contact with the
rolling elements (figure 8 ). Several fracture surfaces showed
evidence of fatigue cracking. The external surfaces of the bearing
outer race showed fretting corrosion and wear to the extent of
seating within the pitch change spider assembly ( figure 9). There
was no evidence of circumferential scoring or other indications of
race rotation within the housing or on the bearing seat. Traces of
light oil were found on the bearing seat. The odour and appearance
of the oil were typical of hydraulic fluid.

Bearing construction

The ATSB examined the tail rotor pitch change bearings from two
other AS332L helicopters maintained by the same operator. Both of
those bearings and their integral seals were found to be in
serviceable condition and showed none of the characteristic
indications of failure presented by the bearing from VH-BHY. The
service lives of both examined bearings were comparable to the
failed unit from VH-BHY. A sample of grease from one of the
serviceable bearings was subject to a solubility test with a small
quantity of hydraulic fluid recovered from the tail rotor servo
fitted to VH-BHY at the time of the incident. With a small amount
of manual agitation, the grease proved miscible within the
hydraulic fluid, producing a liquid with a characteristic viscosity
not appreciably greater than the original hydraulic fluid. Weighing
the bearing before and after cleaning found the unit carrying 1.88
grams of grease, which the aircraft manufacturer indicated was a
nominal quantity.

Bearing contaminant analysis

Samples of the remnant lubricant from inside the failed bearing,
the uncontaminated grease from a serviceable bearing and the
hydraulic fluid from VH-BHY were forwarded to an analytical
laboratory to determine whether any trace of the hydraulic fluid
could be detected within the material from the failed bearing.

Results from that analysis confirmed the presence of
characteristic spectral peaks from the hydraulic fluid to exist
within the remnants of the grease from the failed bearing. These
peaks did not exist within the sample of uncontaminated grease from
the serviceable bearing.

Analysis

The investigation found that the loss of tail rotor control
reported by the flight crew of VH-BHY occurred as a result of the
disconnection of the tail rotor pitch change servo from the control
rod. That disconnection was a direct result of a breakdown in the
anti-friction properties of the tail rotor pitch change shaft
bearing, allowing the rotational torque along the pitch change
shaft to overcome the assembly torque and locking assembly of the
servo end shaft nut. The rotating shaft subsequently unscrewed the
nut, allowing it to drop into the tail structure from where it was
recovered.

The tail rotor pitch change shaft bearing failure occurred as a
result of the contamination and dilution of the grease lubricant,
leading to the internal mechanical breakdown of the bearing cage
and the partial seizure of the assembly. Testing showed that the
bearing grease was contaminated by hydraulic fluid, which likely
released from a leaking tail rotor servo-actuator unit identified
and replaced 22 days before the incident. The bearing was inspected
at the time of the leak discovery and was found to be satisfactory
for further service. At that time, there was no requirement to
change the bearing in the event of the leakage of hydraulic fluid
into the bearing space.

Significant Factors

The following factors were identified as significant to the
development of the incident.

The tail rotor control servo unit developed a hydraulic fluid
leak, with some of the lost fluid entering the pitch change shaft
bearing space.

Migration of hydraulic fluid into the bearing diluted the
grease, affecting the lubricant efficacy and producing accelerated
wear and break-up of the bearing cage.

The bearing grease was soluble in the hydraulic fluid.

The bearing was allowed to remain in service following the
discovery and rectification of the hydraulic leak.

The pitch change shaft inboard (servo end) nut was a
conventional ( right-hand) thread, allowing it to be loosened and
unscrewed by torque from the rotating tail rotor drive shaft.

Disconnection of the pitch change shaft from the servo actuator
caused control of the tail rotor to be lost.

Safety Action

Local safety action

Immediately following the incident, the helicopter operator
changed the tail rotor pitch change shaft bearings in all of the
AS332L helicopters in its fleet. Subsequently, the operator issued
Alert Message AM/332/03/008 to all of its local and international
facilities, instructing maintenance personnel to immediately change
the pitch change shaft bearings should they have been contaminated
by hydraulic fluid, or should any doubt exist as to the bearing's
condition.

On 5 November 2003, the aircraft manufacturer issued Information
Telex 00000151, alerting all operators of AS332, AS330 and AS532
aircraft of the subject incident and instructing maintenance
personnel to check the tail rotor pitch change bearing if any fluid
leak is discovered at the tail rotor servo actuator. Subsequently,
Alert Telex 00000158 was issued on 8 December 2003 requiring
10-hourly inspections for hydraulic leaks at the tail rotor boot
and detailing a new mandatory maintenance procedure to be applied
should a leak be discovered. Due to an error in the original Alert
Telex, an erratum document (Alert Telex 00000166) was issued on 19
December 2003.

On 26 February 2004, the Direction Gnrale de l'Aviation Civile
France ( DGAC) issued airworthiness directive (AD) No. F-2004-031,
mandating the 10-hourly inspection of the AS332 tail rotor hub boot
for evidence of hydraulic fluid leakage. If fluid leakage is
discovered, replacement of the pitch-change shaft bearing is
required. The Australian Civil Aviation Safety Authority
subsequently issued AD/S-PUMA/51 on 26 February 2004, mirroring the
requirements of AD F-2004-031 for aircraft in the Australian
fleet.